The present invention relates to a statistical multiplexing method and apparatus in a packet communication system and, more particularly, to a method and apparatus of efficiently multiplexing a high speed packet signal to a transmission line of a large capacity.
In the packet switching system, a packet multiplexer for statistically multiplexing signals from a plurality of terminals is used. The packet multiplexer is provided between the terminal equipment and the packet switching system and temporarily stores packet signals into a memory from a plurality of terminal equipment and sequentially transmits the packet signals to the switching system.
An example of such a statistical multiplexer has been disclosed in the literature by Marek R. Wernik, "Architecture and Technology Considerations for Multimedia Broadband Communications", Proceedings of Global Telecommunications Conference, November, 1988, pp. 663-667.
Two kinds of effects are obtained in the signal multiplex. The first effect is the improvement of the efficiency of the transmission line. The signals transmitted from the terminal equipment do not always exist at a time when there is no signal to be transmitted from the multiplexer. At such a time, the other terminal equipment may use the transmission line, so that the utilization of the multiplexing transmission line can be increased. Second, the transmission cost per terminal equipment can be reduced by sharing the transmission line with other terminal equipment. The line costs can be reduced by transmitting the signals from a plurality of terminal equipment in a lump rather than by directly transmitting the signals from terminal equipment to the switching system.
A packet multiplexer is used to obtain the first effect. A circuit multiplexer is used to obtain the second effect. The circuit multiplexer is different from the packet multiplexer, in that the timing positions in the secondary circuit (multiplexing circuit) of all of the primary circuits are deterministically assigned in the circuit multiplexer. Hitherto, the circuit multiplexer is generally used for multiplexing to the transmission line which has a large capacity. Such a method is based on a feature in which as the circuit multiplex executes the deterministic operation, the high speed operation can be performed as compared with the packet multiplex.
The statistical multiplexing is executed by multiplexing to the primary circuits. Such a multiplexing method is effective where the multiplexed circuit speed is sufficiently higher than the packet signal speed. The packet signals from the terminal equipment are statistically multiplexed to the primary circuits of the circuit multiplexer by the packet multiplexer. The circuit multiplexer deterministically multiplexes the primary circuits to the secondary circuit of a large capacity.
In a conventional packet multiplexer, it is presumed that the speed of the packet signals is sufficiently lower than the speed of the packet multiplexing signal. For instance, the packet signal speed could be set to 64 kb/sec while the packet multiplexing signal speed is set to 1.5 Mb/sec, and the ratio between those speeds is ten times or more.
It has been disclosed in the above-mentioned reference literature that even in the case where high speed signals such as video information or file information of a large capacity are assembled for packets and are multiplexed, in order to obtain the similar statistic effects, it is necessary that the multiplexing circuit speed is set to be higher than the high speed packet signal speed by ten times or more.
However, if the packet signal speed is set to be 1/10 or more of the primary circuit speed, as mentioned above, no statistic effect is derived and such a case is equivalent to that the circuits of the speed similar to the packet signal speed are deterministically assigned. Further, if the packet signals of a speed which is set to be 1/2 or more of the primary circuit speed are input from the terminal equipment which are packet multiplexed to the same primary circuit, the other packet signals cannot be transmitted in spite of the fact that almost of the secondary circuit is usable.
In the case where the packet signals of the speed similar to the primary circuit speed of such a circuit multiplexer are statistically multiplexed, the signal processes substantially similar to that in the secondary circuit of the circuit multiplexer are requested. To cope with such signals of a large capacity, the fairly higher signal processing speed than that in the conventional packet multiplexer is requested. Such a method of statistically multiplexing by the packet multiplexer is generally not practical from a view point of the multiplexing costs.